The present disclosure describes systems and techniques relating to semiconductor-based imaging devices, for example, a photodiode-based complementary metal oxide semiconductor (CMOS) imager.
Traditional image sensors, such as Charge Coupled Devices (CCDs) and CMOS image sensors, have been widely used for many different applications. In general, CMOS imagers can provide low-cost, low-power, reliable, highly integrated and miniaturized imaging systems. In addition, the availability of sub-micron processing technology, coupled with the advent of active pixel imaging concepts, has led to the development of high performance CMOS imagers.
CCD image sensors generally require more power than CMOS image sensors, are frequently more expensive, and are generally more limited in high-speed operation capabilities and signal handling capacity. In space-based applications, CCDs can generate centroiding errors due to radiation-hit, variable smear, and large power dissipation. However, CCDs are commonly used in many applications due to other advantages over CMOS imagers. For example, CCDs are commonly used in space-based applications, such as in space guidance and navigation systems, and deep-space optical communication systems, which require accurate and stable beam pointing for high speed data transfer. CCDs are frequently used in space-based applications because of their wide availability (CCDs have been used in space imaging applications since the 1970s), relatively low noise, fairly uniform response, and large dynamic range characteristics.
CCD chips are frequently used in star tracking applications. A star tracker is used by almost all spacecraft to determine three-axis attitude. A star tracker is an electronic camera connected to a computer. Using a sensed image of a portion of the sky, stars can be located and identified, and the orientation of the spacecraft can be determined based on these observations. Traditional CMOS imagers have typically been not well suited to such pointing and tracking applications.
Star trackers have been commercially available for more than a decade. Commercial star trackers typically have a mass of a few kilograms and a power consumption of approximately 10 Watts. Commercial star trackers can be made radiation tolerant up to approximately 100 KRads. Moreover, with a Charge Injection Device (CID) and extensive shielding, a star tracker can be made to withstand very high radiation levels (e.g., potentially up to 4 Mrads). However, CID chips tend to be very noisy, and are generally only desirable in extreme radiation environments.